The present invention relates to the field of deacidification of a gaseous effluent. More precisely the present invention proposes compounds for reducing the degradation of an absorbent solution employed for absorbing the acidic compounds contained in a gaseous effluent, the absorbent solution comprising amines in aqueous solution. In particular, the invention relates to compounds used for reducing the degradation of the amines used for the deacidification of gases containing oxygen, for example flue gas.
The deacidification of gaseous effluents, such as natural gas and flue gas for example, is generally carried out by scrubbing with an absorbent solution. The absorbent solution makes it possible to absorb the acidic compounds present in the gaseous effluent (H2S, mercaptans, CO2, COS, SO2, CS2).
The deacidification of these effluents, notably decarbonation and desulfurization, imposes specific constraints on the absorbent solution, in particular thermal and chemical stability notably in the presence of the impurities of the effluent, i.e. essentially oxygen, SOx and NOx. Oxygen can also come into contact with the absorbent solution without necessarily being present in the gaseous effluent to be treated, as in the case for example of accidental ingress of air at the absorbent solution storage tanks.
The absorbent solutions most used at present are aqueous solutions of alkanolamines. We may mention document FR 2 820 430, which proposes methods for deacidifying gaseous effluents.
However, a person skilled in the art is well aware that these amines have the drawback that they undergo degradation in the service conditions.
In particular, the amines can be degraded by oxygen, causing consumption of the amine and the formation of degradation products, which accumulate in the unit or, for the most volatile products, are entrained in the gaseous effluents from the process. Thus, especially in the case of post-combustion treatment of flue gas in a method using an aqueous solution of monoethanolamine (MEA), large amounts of ammonia are formed. The ammonia thus formed is entrained into the atmosphere with the treated flue gas, which poses problems with respect to protection of the environment.
In the case of capture of the CO2 in the flue gases from industrial units or from production of electricity or of energy in general, the phenomena of degradation of the absorbent solution based on amines are increased by the presence of a massive amount of oxygen in the feed to be treated, which can generally be up to 5 vol. %. In the case of flue gas from the natural gas combined cycle, the content of oxygen by volume in the flue gas can reach 15%.
The degraded solution is characterized by:                a decrease in absorption of the acidic compounds in the feed relative to a fresh amine solution,        an increase in the density of the absorbent solution, as well as its viscosity, which can lead to a loss of performance,        formation of more-volatile amines that contaminate the treated gas and the acidic gas from the regeneration step: ammonia, methylamine, dimethylamine and trimethylamine for example, depending on the nature of the amine used,        accumulation of degradation products in the absorbent solution, which may necessitate treatment of the degraded solution,        possible problems with foaming due to the degradation products.        The degradation of the absorbent solution therefore affects the performance and proper functioning of the gas deacidification units.        
To overcome the problem of degradation, if it is not possible to limit or eliminate the presence of oxygen in the absorbent solution, compounds are added to the absorbent solution whose role is to prevent or limit the phenomena of degradation of the amine compounds, notably the degradation caused by oxidation phenomena. These compounds are commonly called degradation inhibiting additives. The principal known modes of action of the degradation inhibiting additives consist, depending on their nature, of a reaction of the reduction type and/or of capture, trapping and/or stabilization of the radicals formed in the absorbent solution in order to limit or prevent or interrupt the reactions, notably the chain reactions, of degradation.
U.S. Pat. No. 5,686,016 mentions additives used for limiting the degradation of absorbent solutions used for the deacidification of natural gas, in particular oximes.
U.S. Pat. No. 7,056,482 mentions additives used for limiting the degradation of absorbent solutions used for capture of CO2, in particular thiosulfates and sulfites.